Solid state converter for high voltages

1. A converter for producing an alternating voltage output waveform from a high input voltage, the apparatus comprising: a) a bridge circuit having switching assemblies so connected as to define at least two input nodes and two output nodes, the input nodes being operative for receiving the high input voltage across the input nodes, the output nodes being operative for providing a take-off point for the output waveform, each switching assembly comprising at least two high power semiconductor switching devices connected in series between a respective input node and a respective output node to define a respective arm of the bridge circuit, and a capacitor connected in parallel with each of the switching devices in each arm of the bridge circuit; b) a resonant circuit having a resonant frequency and connected across the output nodes; and c) a control circuit for supplying a control waveform to each switching device in the bridge circuit, the control circuit being operative for generating control signals to switch the switching devices of the switching assemblies in each arm between on and off states in a pattern having a period substantially equal to the resonant frequency of the resonant circuit to provide the output waveform across the output nodes with a frequency which is substantially at the resonant frequency, the control circuit being operative for switching each device in the respective arm of the bridge circuit from the on state to the off state for one half cycle of a current in the resonant circuit, and from the off state to the on state for a remaining one half cycle, the switching being performed substantially at the resonant frequency of the resonant circuit.

2. The converter according to claim 1 , in which at least ten semiconductor switching devices are connected in series in each arm of the bridge circuit.

3. The converter according to claim 1 , in which each capacitor has a capacitance value selected to determine a rate of change of voltage across a respective switching device when switching from the on state to the off state and vice versa.

4. The converter according to claim 3 , in which all the capacitors across the switching devices in a respective arm of the bridge circuit have substantially the same capacitance value.

5. The converter according to claim 3 , in which the control circuit is operative for controlling all of the switching devices in the bridge circuit so that all of the switching devices are held in the on state substantially as the voltage across the output nodes crosses zero so as to discharge the capacitors connected across the switching devices, before at least one of the switching devices is switched to the off state.

6. The converter according to claim 5 , in which the control circuit is operative for controlling the switching devices so that all of the switching devices are switched to the on state for a predetermined period of time whenever the voltage across the resonant circuit crosses zero.

7. The converter according to claim 1 , in which the control circuit comprises a central processing unit that generates a single command signal and modifies it to produce the control signals which respectively determine the on and off states of every switching device in a respective arm of the bridge circuit.

8. The converter according to claim 1 , in which the resonant circuit comprises an inductive element which is connected in parallel with a capacitive element between the output nodes.

9. The converter according to claim 1 , in which the control circuit is operative for controlling the switching devices so that each of the switching devices of the switching assemblies switches from the on state to the off state, and vice versa, substantially as a voltage across the resonant circuit, and hence the output nodes, crosses zero.

10. The converter according to claim 1 , in which an inductor is connected in series between at least one of the input nodes and the switching devices that are connected to said at least one input node.

11. The converter according to claim 1 , and further including an isolating transformer having a primary winding and at least one secondary winding, the primary winding being connected across the output nodes, and the secondary winding providing an output from which the output waveform is taken.

12. A converter for converting a steady state high dc input voltage into an alternating voltage output waveform, the converter comprising: a) a bridge circuit having switching assemblies so connected as to define at least two input nodes and two output nodes, the input nodes being operative for receiving the high input voltage across the input nodes, the output nodes being operative for providing a take-off point for the output waveform, each switching assembly comprising at least two high power semiconductor switching devices connected in series between a respective input node and a respective output node to define a respective arm of the bridge circuit, and a capacitor connected in parallel with at least one of the switching devices in each arm of the bridge circuit, there being at least two pairs of arms connected together to define the two input nodes; b) a resonant circuit having a resonant frequency and connected across the output nodes; and c) a control circuit for supplying a control waveform to each switching device in the bridge circuit, the control circuit being operative for generating control signals to switch the switching devices of the switching assemblies in each arm between on and off states in a pattern having a period substantially equal to the resonant frequency of the resonant circuit to provide the output waveform across the output nodes with a frequency which is substantially at the resonant frequency, the control circuit being operative for switching each device in the respective arm of the bridge circuit from the on state to the off state for one half cycle of a current in the resonant circuit, and from the off state to the on state for a remaining one half cycle, the switching being performed substantially at the resonant frequency of the resonant circuit.

13. A converter for converting a multiple phase alternating input voltage waveform having a frequency into a single phase alternating voltage output waveform, the converter comprising: a) a bridge circuit having switching assemblies so connected as to define at least two pairs of input nodes and two output nodes, each of the two pairs of input nodes being operative for receiving the input voltage waveform from one phase of a supply across the input nodes, and the output nodes being operative for providing a take-off point for the single phase alternating voltage output waveform, each switching assembly comprising at least two high power semiconductor switching devices connected in series between a respective input node and a respective output node to define a respective arm of the bridge circuit; b) a resonant circuit having a resonant frequency and connected across the output nodes; and c) a control circuit for supplying a control waveform to each switching device in the bridge circuit, the control circuit being operative for generating control signals which switch the switching devices of the switching assemblies in each arm between on and off states in a pattern having a period substantially equal to the resonant frequency of the resonant circuit to provide the output waveform across the output nodes at a frequency which is substantially at the resonant frequency, and the resonant frequency being greater than the frequency of the input waveform, the control circuit being operative for switching each device in the respective arm of the bridge circuit from the on state to the off state for one half cycle of a current in the resonant circuit, and from the off state to the on state for a remaining one half cycle, the switching being performed substantially at the resonant frequency of the resonant circuit.

14. An apparatus for producing an alternating voltage output waveform from an input voltage, the apparatus comprising: a) a bridge circuit having switching assemblies so connected as to define at least two input nodes and two output nodes, the input nodes being operative for receiving the input voltage across the input nodes, the output nodes being operative for providing a take-off point for the output waveform, each switching assembly comprising at least two semiconductor switching devices connected in series between a respective input node and a respective output node to define a respective arm of the bridge circuit, and a capacitor connected in parallel with at least one of the switching devices in each arm of the bridge circuit; b) a resonant circuit having a resonant frequency and connected across the output nodes; and c) a control circuit for supplying a control waveform to each switching assembly in the bridge circuit, the control circuit being operative for generating control signals to switch the switching devices of the switching assemblies in each arm between on and off states in a pattern having a period substantially equal to the resonant frequency of the resonant circuit to provide the output waveform across the output nodes with a frequency which is substantially at the resonant frequency, the control circuit being operative for switching each device in the respective arm of the bridge circuit from the on state to the off state for one half cycle of a current in the resonant circuit, and from the off state to the on state for a remaining one half cycle, the switching being performed substantially at the resonant frequency of the resonant circuit.